This invention relates generally to an electrophotographic printing device, and more particularly, a cleaning blade used therein to remove particles adhering to the photoconductive member.
In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is imagewise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a sheet of support material and permanently affixed thereto.
This process is well known and useful for light lens copying from an original and printing applications from electronically generated or stored originals, and in ionography.
In a reproduction process of the type as described above, it is inevitable that some residual toner will remain on the photoconductive surface after the toner image has been transferred to the sheet of support material (e.g. paper). It has been found that with such a process that the forces holding some of the toner particles to the imaging surface are stronger than the transfer forces and, therefore, some of the particles remain on the surface after transfer of the toner image. In addition to the residual toner, other particles, such as paper debris (i.e. Kaolin, fibers, clay), additives and plastic, are left behind on the surface after image transfer. (Hereinafter, the term "residual particles" encompasses residual toner and other residual debris remaining after image transfer.) The residual particles adhere firmly to the surface and must be removed prior to the next printing cycle to avoid its interfering with recording a new latent image thereon.
Various methods and apparatus may be used for removing residual particles from the photoconductive imaging surface. Hereinbefore, a cleaning brush, a cleaning web, and a cleaning blade have been used. Both cleaning brushes and cleaning webs operate by wiping the surface so as to affect transfer of the residual particles from the imaging surface thereon. After prolonged usage, however, both of these types of cleaning devices become contaminated with toner and must be replaced. This requires discarding the dirty cleaning devices. In high-speed machines this practice has proven not only to be wasteful but also expensive.
The shortcomings of the brush and web made way for another now prevalent form of cleaning known and disclosed in the art--blade cleaning. Blade cleaning involves a blade, normally made of a rubberlike material (e.g. polyurethane) which is dragged or wiped across the surface to remove the residual particles from the surface. Blade cleaning is a highly desirable method, compared to other methods, for removing residual particles due to its simple, inexpensive structure. However, there are certain deficiencies in blade cleaning, which are primarily a result of the frictional sealing contact that must occur between the blade and the surface.
Dynamic friction is the force that resists relative motion between two bodies that come into contact with each other while having separate motion. This friction between the blade edge and the surface causes wearing away of the blade edge, and damages the blade's contact with the surface. For purposes of this application, volume wear (W) is proportional to the load (F) multiplied by the distance (D) traveled. Thus, W.varies.FD.varies.FVT, or introducing a factor of proportionality K, W=KFVT where K is the wear factor, V is the velocity and T is the elapsed time. Hence, wear increases with larger values of K. Various blade lubricating materials or toner lubricant additives have been proposed to reduce friction which would thereby reduce wear. However, lubricants tend to change the operational characteristics of the printing machine undesirably. For example, a polyurethane blade with a good lubricant in the toner can ideally achieve a frictional coefficient of about 0.5, however, this rarely occurs because of the delicate balance involved in achieving the proper weight percent of lubricant in the toner. (Normal frictional coefficient values for cleaning blades that remove toner off the imaging surface range from a low of about 0.5 to a high of about 1.5).
In addition to the problem of volume wear, blades are also subject to unpredictable failures. In normal operational configuration, with a coefficient of dynamic friction in the range of about 0.5 to about 1.5, a blade cleaning edge or tip in sealing contact with the surface is tucked slightly. The blade is not in intimate contact with the surface, but slides on toner particles and lubricant to maintain the sealing contact required for cleaning. In this configuration, the blade may flatten particles that pass under the blade and cause impaction of particles on the surface. This is called cometing because of the comet-like impressions created by the flattened particles. Also, the carrier beads remaining on the surface subsequent to development may damage the blade. Another common failure is localized increases in friction between the blade and surface that cause the phenomenon of severe tucking, where the blade cleaning edge becomes tucked underneath the blade. When this occurs the cleaning blade material can fracture in the region where the severe tuck occurs and damage the blade permanently. Still another common failure occurs at startup, when the frictional force between the blade and the surface is so high that it causes the blade to foldover on itself overstressing the blade. These types of failures require removal and replacement of the blade.
The commonly used elastomer-type cleaning blade is a resilient material that allows stubborn residual particles to remain on the surface. This occurs because the resilient elastomeric material is unable to provide sufficient contact to create a tight seal between the cleaning blade and the surface when tuck occurs, therefore the resiliency of the elastomeric blade makes it easy for the blade to glide over the residual particles. It is an object of this invention to provide a sufficiently abrasive blade surface to remove these stubborn residual particles and thus avoid the resiliency problem of the elastomer-type blade.
While it might appear that a rigid metal blade might solve the problems of rigidity and wear, in fact, the frictional contact required between the surface and blade quickly wears away the blade and any surface lubricants applied thereto. As the blade edge wears, it changes from a chiseling edge to a rounded or flattened surface which requires a high force to maintain the edge in sealing contact. While a beveled edge is useful in liquid toner applications, it is highly susceptible to damage and wear in dry toner applications. Accordingly, it is desirable to maintain the blade's square edge without wear. Additionally, wearing friction may generate toner fusing temperatures, causing toner to fuse to the blade, or the surface. Furthermore, filming on the surface can deteriorate image quality. Filming occurs either uniformly or as streaking, due to deficiencies in blade cleaning, requiring the use of a lubricant and a balancing abrasion element to prevent filming.
A current focus in the market place is on intelligent machines. These are intelligent in the way that they operate; meaning that they monitor their own performance and adapt to changing conditions. They are also perceived to be intelligent by their operators due to their ability to understand and satisfy the operators' objectives. An implicit requirement in the perception of intelligence is highest reliability, which translates to minimizing unscheduled maintenance and machine shut downs.
Concepts for these intelligent machines (a) enable a reliability improvement over the way machines are currently built, either through a self-sensing scenario or through inherently more reliable design, or (b) provide a feature that the operator would interpret as machine intelligence. The present invention is one such concept: Blade cleaners are often used in xerographic systems to remove toner from the photoreceptor which has not transferred onto the paper. It is also called upon to clean off the entire image in the event that a sheet of paper did not reach the transfer point in time to meet the image and a shutdown or jam is declared. Since the cleaner blade must remain continuously in contact with the photoreceptor belt it is essential that all of the critical blade design parameters (e.g. stiffness, angle, load etc.) be selected within an operating window that is bounded by some photoreceptor life on the other. The loading stiffness, etc., must be adequate to remove the normal toner residue but it will occasionally encounter toner agglomerates, foreign contaminants and impacted particles which will exceed the blades capability to clean. These particles will generally cause copy defects which are objectionable. It is an object of this invention to remove these stubborn particles that normal cleaning will not remove.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 3,843,407 to Thorp describes a method and apparatus for blade cleaning of an imaging surface which is cleaned by contacting a blade while moving in the normal direction and temporarily reversing the direction of the imaging surface relative to the cleaning blade while maintaining the same contact of the cleaning blade with the imaging surface.
U.S. Pat. No. 3,940,282 to Hwa describes cleaning of reusable surfaces by blades with relative motion between an imaging surface and a blade. The motion of the imaging surface is reversed with the blade still engaged with imaging surface before each rest period.
U.S. Pat. No. 4,264,191 to Gerbasi et al. describes a laminated doctor blade for removing excess marking material or other material from a surface. The blade comprises a relatively hard layer of a smooth tough material and a relatively soft layer of resilient material.